Download (3249Kb) - D-Scholarship@Pitt - University of Pittsburgh

of 321 to 322 in 37 % yield when the reaction was performed under Ar atmosphere (Scheme
4.51). bb In this experiment 322 was formed with high diastereoselectivity (>10 : 1) and high
regioselectivity, since only trace amounts of the 4-alkylidene cyclopentenone were observed.
Similarly, the methyl-substituted allenyne 74c, resulted in selective formation of α-alkylidene
cyclopentenones in 43% yield (7 : 1 diastereomeric ratio). Since both reactions were performed
under Ar atmosphere, the yield around 40% suggested that the reaction is stoichiometric in Rh(I),
and the carbon monoxide incorporated in the products results from the starting catalyst (10 mol
% [Rh(CO)2Cl]2 contains 40 mol % CO).
Scheme 4.51 Rh(I)-mediated formation of α-alkylidene cyclopentenones.
BzN
MeO 2C
BzN
MeO 2C
•
H
H
10 mol % [Rh(CO) 2Cl] 2
20 mol% dppb, 22 mol % AgBF 4
DCE/tol (9 : 1), Ar, rt, 30 min
37%, dr > 10 : 1
BzN O
MeO 2C
321 322
•
H
H
10 mol % [Rh(CO) 2Cl] 2
20 mol% dppb, 22 mol % AgBF 4
DCE/tol (9 : 1), Ar, 50 o C, 2h
43%
BzN O
MeO 2C
74c 288c : 287c = ~ 7 : 1
dppb =
Ph
P Ph
P Ph
Ph
Although these results were encouraging, attempts to perform the reaction catalytically under CO
atmosphere (1 atm, balloon pressure) gave only recovered starting material and no
cyclocarbonylation products if CO atmosphere is introduced to the system prior to addition of the
allenyne. Furthermore, reproducibility issues were encountered, mainly due to the multiple
components required for preparation of the active Rh-species. cc It was also found that the
bb Use of dppm and dppe in the same experiment gave predominantly the 4-alkylidene cyclopentenone, while
BINAP unselectively gave mixtures of products in a very sluggish reaction.
cc In particular, the amount of dppb relative to [Rh(CO)2Cl]2 proved critical for obtaining high regio- and
diastereoselectivity. Decreasing the amount of dppb below 20 mol% led to unselective reaction with both double
bonds.
132